This invention relates to a thread structure for interconnecting two elements in a percussion drill string comprising an interior cylindrical thread and an exterior cylindrical thread, said threads having abutting flanks which are straight along their abutting contact portions.
A thread structure of this type is described in U.S. Pat. No. 3,645,570. In such a thread structure friction martensite is formed at the contact surfaces resulting in so-called pittings. This is particularly the case when compressed air is used as flushing medium which leads to a comparatively poor cooling of the contact surfaces, and when the contact surfaces are carburized. When these so-called pittings do arise on the flanks of the threads they are comparatively harmless except in the bottom portion of the exterior thread, however, where the stress concentration is high, and a surface defect does rapidly lead to fatigue failure of the drill element.
The object of the present invention is to provide a thread structure of the above-described type in which it is ensured that contact will not arise between the crest portion of the interior thread and the bottom portion of the exterior thread when the threads are worn.
Specifically, this invention seeks to provide an improved coupling thread on a drill rod which is exteriorly threaded at its one end and interiorly threaded at its other end, where the interior thread is provided in a sleeve-shaped portion of the drill rod. A drill rod of this type is described in U.S. Pat. No. 3,537,738.
Other objects of the invention are to provide a drill rod of the type described in U.S. Pat. No. 3,537,738 having the same bending resistance throughout its length and to ensure that flushing medium can reach the threads to flow therebetween for purposes of decreasing the wear of the threads.